1. Field of the Invention
The present invention relates to an optical linear encoder which is used as a position detection means for detecting the position of, e.g., a carriage that reciprocally moves while carrying a recording head, and a recording apparatus using the same.
2. Related Background Art
Conventionally, various types of linear encoders are used as high-precision position detection means for precisely detecting the position of, e.g., a carriage that reciprocally moves in a recording apparatus. Of these encoders, an optical encoder has high durability, is strong against disturbance such as changes in magnetic field, is inexpensive, and is suitable for high-resolution position detection since its detection unit can perform detection without contacting a linear scale. Accordingly, the optical encoder is popularly used in high-performance recording apparatuses.
However, recently, as requirements for higher performance of the apparatus become stricter, the optical linear encoder is required to have higher precision and higher resolution, and hence, the following problems are encountered.
Inside the recording apparatus, there are many contaminants such as ink, paper powder, toner, and the like, and when such foreign matter becomes attached to the surface of the linear scale, transmission light decreases, thus lowering the signal output of the linear encoder. Japanese Laid-Open Patent Application No. 1-221271 proposed a means for controlling the total sensitivity of a photointerrupter to a predetermined value in accordance with the photointerrupter output upon scanning a linear scale. However, this means can correct slight changes in signal, but cannot solve the problem of output drop due to contamination of the scale surface. Even when the degree of contamination remains the same, such contamination influences a high-resolution encoder more seriously since the encoder has higher sensitivity.
In particular, in an ink-jet type recording apparatus, the phenomenon of scattering a small amount of ink as mist into a space upon ejecting ink from a recording head (such phenomenon will be referred to as "mist" hereinafter) is experienced, and the ink becomes readily attached to parts inside the apparatus and, especially, to the linear scale surface.
Recently, not only an optical linear encoder that simply counts the number of light and dark slits using a light-receiving element but also an encoder that attains high-resolution, high-precision detection by combining optical phenomena such as reflection, refraction, and the like are available. In such encoder, a contaminant such as a water droplet, which transmits light and serves to change the curvature of the linear scale surface, deflects transmission light of the scale from the normal optical path. Consequently, an error signal that cannot be corrected by gain adjustment is generated.
Errors caused by the error signal influence the print performance of a recording apparatus that uses the linear encoder, and accumulation of errors due to the error signals brings about stop position errors. In the worst case, a carriage which is driven at high speed may collide against other parts, and may result in mechanical damage, runaway, and the like, thus lowering the reliability and durability.